Istanbul is a port city.  There are always ships coming and going. It is also the gateway to the Black Sea and many ships pass through the straights heading north and south. There are also many sea-taxis which will take you to islands or other nearby ports. I spent part of one day island hoping and on one of the ships. The whole day's transportation cost about $2.50. The entry token was interesting because it had little grooves on on it and you had to line then up with the slots when you dropped them into the machine. It was fascinating to watch all the different types of people crowd onto the ships. This was likely my favorite part of Istanbul.